uboot組織架構正在朝著linux架構方向發展，不同版本稍有不同，一下以U-Boot 2016.03為例。

分析入口：以u-boot.lds（其決定了各個段的排布方式）開始：

1、u-boot.lds:

//設定輸出檔案大小端格式
OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
//設定檔案以 ARM 可執行檔案格式輸出
OUTPUT_ARCH(arm)
//指令以_start標號開始
ENTRY(_start)
SECTIONS
{
 . = 0x00000000;
 . = ALIGN(4);
 .text :
 {
  *(.__image_copy_start)
  *(.vectors)
//text 段以start.o開始
  arch/arm/cpu/armv7/start.o (.text*)
  //其他text段
  *(.text*)
 }
 . = ALIGN(4);
//只讀資料段，首先將所有檔案的只讀資料段以名字（即 asc 碼排序），然後以對齊方式排序存放到 text 段後面。
 .rodata : { *(SORT_BY_ALIGNMENT(SORT_BY_NAME(.rodata*))) }
 . = ALIGN(4);
//所有的資料段
 .data : {
  *(.data*)
 }
. = ALIGN(4);
 . = .;
 . = ALIGN(4);
 .u_boot_list : {
  KEEP(*(SORT(.u_boot_list*)));
 }
 . = ALIGN(4);
 .image_copy_end :
 {
  *(.__image_copy_end)
 }
 .rel_dyn_start :
 {
  *(.__rel_dyn_start)
 }
 .rel.dyn : {
  *(.rel*)
 }
 .rel_dyn_end :
 {
  *(.__rel_dyn_end)
 }
 .end :
 {
  *(.__end)
 }
 _image_binary_end = .;
 . = ALIGN(4096);
 .mmutable : {
  *(.mmutable)
 }
 .bss_start __rel_dyn_start (OVERLAY) : {
  KEEP(*(.__bss_start));
  __bss_base = .;
 }
 .bss __bss_base (OVERLAY) : {
  *(.bss*)
   . = ALIGN(4);
   __bss_limit = .;
 }
 .bss_end __bss_limit (OVERLAY) : {
  KEEP(*(.__bss_end));
 }
 .dynsym _image_binary_end : { *(.dynsym) }
 .dynbss : { *(.dynbss) }
 .dynstr : { *(.dynstr*) }
 .dynamic : { *(.dynamic*) }
 .plt : { *(.plt*) }
 .interp : { *(.interp*) }
 .gnu.hash : { *(.gnu.hash) }
 .gnu : { *(.gnu*) }
 .ARM.exidx : { *(.ARM.exidx*) }
 .gnu.linkonce.armexidx : { *(.gnu.linkonce.armexidx.*) }
}
 

通過u-boot.lds我們知道，第一個執行的地方是_start，它在arch/arm/lib/vector.S中被定義。

2、vector.S：_start

檔案路徑：arch/arm/lib/vector.S

_start:

#ifdef CONFIG_SYS_DV_NOR_BOOT_CFG
	.word	CONFIG_SYS_DV_NOR_BOOT_CFG
#endif

	b	reset
	ldr	pc, _undefined_instruction
	ldr	pc, _software_interrupt
	ldr	pc, _prefetch_abort
	ldr	pc, _data_abort
	ldr	pc, _not_used
	ldr	pc, _irq
	ldr	pc, _fiq
 

通過_start我們知道，其之後跳轉到了reset，其在arch/arm/cpu/armv7/start.S中被定義。

3、start.S：reset

檔案位置：arch/arm/cpu/armv7/start.S

#include <asm-offsets.h>
#include <config.h>
#include <asm/system.h>
#include <linux/linkage.h>
/*************************************************************************
 * Startup Code (reset vector)
* Do important init only if we don't start from memory!
 * Setup memory and board specific bits prior to relocation.
 * Relocate armboot to ram. Setup stack.
*************************************************************************/
	.globl	reset
	.globl	save_boot_params_ret

reset:
	/* Allow the board to save important registers */
	b	save_boot_params
save_boot_params_ret:
	/*
	 * disable interrupts (FIQ and IRQ), also set the cpu to SVC32 mode,
	 * except if in HYP mode already
	 */
	//設定CPSR暫存器裡設定CPU為SVC模式，禁止中斷
	mrs	r0, cpsr
	and	r1, r0, #0x1f		@ mask mode bits
	teq	r1, #0x1a		@ test for HYP mode
	bicne	r0, r0, #0x1f		@ clear all mode bits
	orrne	r0, r0, #0x13		@ set SVC mode
	orr	r0, r0, #0xc0		@ disable FIQ and IRQ
	msr	cpsr,r0
/*
 * Setup vector:
 * (OMAP4 spl TEXT_BASE is not 32 byte aligned.
 * Continue to use ROM code vector only in OMAP4 spl)
 */
#if !(defined(CONFIG_OMAP44XX) && defined(CONFIG_SPL_BUILD))
	/* Set V=0 in CP15 SCTLR register - for VBAR to point to vector */
	mrc	p15, 0, r0, c1, c0, 0	@ Read CP15 SCTLR Register
	bic	r0, #CR_V		@ V = 0
	mcr	p15, 0, r0, c1, c0, 0	@ Write CP15 SCTLR Register

	/* Set vector address in CP15 VBAR register */
	ldr	r0, =_start
	mcr	p15, 0, r0, c12, c0, 0	@Set VBAR
#endif

	/* the mask ROM code should have PLL and others stable */
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
//呼叫cpu_init_cp15，初始化協處理器CP15,從而禁用MMU和TLB
	bl	cpu_init_cp15
	//呼叫cpu_init_crit，進行一些關鍵的初始化動作，也就是平臺級和板級的初始化
	bl	cpu_init_crit
#endif
//跳轉到主函式，也就是板級初始化函式
	bl	_main
 

通過start.S我們知道，其之後分別執行了cpu_init_cp15（本檔案中定義）和cpu_init_crit（在arch\arm\cpu\armv7\ lowlevel_init.S中定義）。

4、lowlevel_init.S：cpu_init_crit

檔案位置：arch\arm\cpu\armv7\ lowlevel_init.S

其中跳轉執行s_init，在arch\arm\cpu\armv7\mx6\ soc.c中定義。

之後結束了cpu_init_crit的執行，會返回到start.S中執行bl  _main，此處也就是第二階段_main在arch\arm\lib\crt0.S中定義。

5、crt0.S：_main

/*
 *  crt0 - C-runtime startup Code for ARM U-Boot
 *
 *  Copyright (c) 2012  Albert ARIBAUD <[email protected]>
 *
 * SPDX-License-Identifier:	GPL-2.0+
 */

#include <config.h>
#include <asm-offsets.h>
#include <linux/linkage.h>
#ifdef CONFIG_CPU_V7M
#include <asm/armv7m.h>
#endif

/*
 * This file handles the target-independent stages of the U-Boot
 * start-up where a C runtime environment is needed. Its entry point
 * is _main and is branched into from the target's start.S file.
 *
 * _main execution sequence is:
 *
 * 1. Set up initial environment for calling board_init_f().
 *    This environment only provides a stack and a place to store
 *    the GD ('global data') structure, both located in some readily
 *    available RAM (SRAM, locked cache...). In this context, VARIABLE
 *    global data, initialized or not (BSS), are UNAVAILABLE; only
 *    CONSTANT initialized data are available. GD should be zeroed
 *    before board_init_f() is called.
 *
 * 2. Call board_init_f(). This function prepares the hardware for
 *    execution from system RAM (DRAM, DDR...) As system RAM may not
 *    be available yet, , board_init_f() must use the current GD to
 *    store any data which must be passed on to later stages. These
 *    data include the relocation destination, the future stack, and
 *    the future GD location.
 *
 * 3. Set up intermediate environment where the stack and GD are the
 *    ones allocated by board_init_f() in system RAM, but BSS and
 *    initialized non-const data are still not available.
 *
 * 4a.For U-Boot proper (not SPL), call relocate_code(). This function
 *    relocates U-Boot from its current location into the relocation
 *    destination computed by board_init_f().
 *
 * 4b.For SPL, board_init_f() just returns (to crt0). There is no
 *    code relocation in SPL.
 *
 * 5. Set up final environment for calling board_init_r(). This
 *    environment has BSS (initialized to 0), initialized non-const
 *    data (initialized to their intended value), and stack in system
 *    RAM (for SPL moving the stack and GD into RAM is optional - see
 *    CONFIG_SPL_STACK_R). GD has retained values set by board_init_f().
 *
 * 6. For U-Boot proper (not SPL), some CPUs have some work left to do
 *    at this point regarding memory, so call c_runtime_cpu_setup.
 *
 * 7. Branch to board_init_r().
 *
 * For more information see 'Board Initialisation Flow in README.
 */

/*
 * entry point of crt0 sequence
 */

ENTRY(_main)

/*
 * Set up initial C runtime environment and call board_init_f(0).
 */

#if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_STACK)
	ldr	sp, =(CONFIG_SPL_STACK)
#else
	ldr	sp, =(CONFIG_SYS_INIT_SP_ADDR)
#endif
#if defined(CONFIG_CPU_V7M)	/* v7M forbids using SP as BIC destination */
	mov	r3, sp
	bic	r3, r3, #7
	mov	sp, r3
#else
	bic	sp, sp, #7	/* 8-byte alignment for ABI compliance */
#endif
	mov	r0, sp
	bl	board_init_f_alloc_reserve
	mov	sp, r0
	/* set up gd here, outside any C code */
	mov	r9, r0
	bl	board_init_f_init_reserve

	mov	r0, #0
	bl	board_init_f

#if ! defined(CONFIG_SPL_BUILD)

/*
 * Set up intermediate environment (new sp and gd) and call
 * relocate_code(addr_moni). Trick here is that we'll return
 * 'here' but relocated.
 */

	ldr	sp, [r9, #GD_START_ADDR_SP]	/* sp = gd->start_addr_sp */
#if defined(CONFIG_CPU_V7M)	/* v7M forbids using SP as BIC destination */
	mov	r3, sp
	bic	r3, r3, #7
	mov	sp, r3
#else
	bic	sp, sp, #7	/* 8-byte alignment for ABI compliance */
#endif
	ldr	r9, [r9, #GD_BD]		/* r9 = gd->bd */
	sub	r9, r9, #GD_SIZE		/* new GD is below bd */

	adr	lr, here
	ldr	r0, [r9, #GD_RELOC_OFF]		/* r0 = gd->reloc_off */
	add	lr, lr, r0
#if defined(CONFIG_CPU_V7M)
	orr	lr, #1				/* As required by Thumb-only */
#endif
	ldr	r0, [r9, #GD_RELOCADDR]		/* r0 = gd->relocaddr */
	b	relocate_code
here:
/*
 * now relocate vectors
 */

	bl	relocate_vectors

/* Set up final (full) environment */

	bl	c_runtime_cpu_setup	/* we still call old routine here */
#endif
#if !defined(CONFIG_SPL_BUILD) || defined(CONFIG_SPL_FRAMEWORK)
# ifdef CONFIG_SPL_BUILD
	/* Use a DRAM stack for the rest of SPL, if requested */
	bl	spl_relocate_stack_gd
	cmp	r0, #0
	movne	sp, r0
	movne	r9, r0
# endif
	ldr	r0, =__bss_start	/* this is auto-relocated! */

#ifdef CONFIG_USE_ARCH_MEMSET
	ldr	r3, =__bss_end		/* this is auto-relocated! */
	mov	r1, #0x00000000		/* prepare zero to clear BSS */

	subs	r2, r3, r0		/* r2 = memset len */
	bl	memset
#else
	ldr	r1, =__bss_end		/* this is auto-relocated! */
	mov	r2, #0x00000000		/* prepare zero to clear BSS */

clbss_l:cmp	r0, r1			/* while not at end of BSS */
#if defined(CONFIG_CPU_V7M)
	itt	lo
#endif
	strlo	r2, [r0]		/* clear 32-bit BSS word */
	addlo	r0, r0, #4		/* move to next */
	blo	clbss_l
#endif

#if ! defined(CONFIG_SPL_BUILD)
	bl coloured_LED_init
	bl red_led_on
#endif
	/* call board_init_r(gd_t *id, ulong dest_addr) */
	mov     r0, r9                  /* gd_t */
	ldr	r1, [r9, #GD_RELOCADDR]	/* dest_addr */
	/* call board_init_r */
#if defined(CONFIG_SYS_THUMB_BUILD)
	ldr	lr, =board_init_r	/* this is auto-relocated! */
	bx	lr
#else
	ldr	pc, =board_init_r	/* this is auto-relocated! */
#endif
	/* we should not return here. */
#endif

ENDPROC(_main)

第二階段主要函式有設定C，載入C環境、board_init_f(common\Board_f.c）、重定位程式碼relocate_code 、執行board_init_r。程式再此處不應該返回，而是陷入死迴圈。

6、Board_f.c：board_init_f

檔案位置：common\Board_f.c

void board_init_f(ulong boot_flags)
{
#ifdef CONFIG_SYS_GENERIC_GLOBAL_DATA
	/*
	 * For some archtectures, global data is initialized and used before
	 * calling this function. The data should be preserved. For others,
	 * CONFIG_SYS_GENERIC_GLOBAL_DATA should be defined and use the stack
	 * here to host global data until relocation.
	 */
	gd_t data;

	gd = &data;

	/*
	 * Clear global data before it is accessed at debug print
	 * in initcall_run_list. Otherwise the debug print probably
	 * get the wrong vaule of gd->have_console.
	 */
	zero_global_data();
#endif

	gd->flags = boot_flags;
	gd->have_console = 0;

	if (initcall_run_list(init_sequence_f))
		hang();

#if !defined(CONFIG_ARM) && !defined(CONFIG_SANDBOX) && \
		!defined(CONFIG_EFI_APP)
	/* NOTREACHED - jump_to_copy() does not return */
	hang();
#endif
}

init_sequence_f：

static init_fnc_t init_sequence_f[] = {
#ifdef CONFIG_SANDBOX
	setup_ram_buf,
#endif
	setup_mon_len,
#ifdef CONFIG_OF_CONTROL
	fdtdec_setup,
#endif
#ifdef CONFIG_TRACE
	trace_early_init,
#endif
	initf_malloc,
	initf_console_record,
#if defined(CONFIG_MPC85xx) || defined(CONFIG_MPC86xx)
	/* TODO: can this go into arch_cpu_init()? */
	probecpu,
#endif
#if defined(CONFIG_X86) && defined(CONFIG_HAVE_FSP)
	x86_fsp_init,
#endif
	arch_cpu_init,		/* basic arch cpu dependent setup */
	initf_dm,
	arch_cpu_init_dm,
	mark_bootstage,		/* need timer, go after init dm */
#if defined(CONFIG_BOARD_EARLY_INIT_F)
	board_early_init_f,
#endif
	/* TODO: can any of this go into arch_cpu_init()? */
#if defined(CONFIG_PPC) && !defined(CONFIG_8xx_CPUCLK_DEFAULT)
	get_clocks,		/* get CPU and bus clocks (etc.) */
#if defined(CONFIG_TQM8xxL) && !defined(CONFIG_TQM866M) \
		&& !defined(CONFIG_TQM885D)
	adjust_sdram_tbs_8xx,
#endif
	/* TODO: can we rename this to timer_init()? */
	init_timebase,
#endif
#if defined(CONFIG_ARM) || defined(CONFIG_MIPS) || \
		defined(CONFIG_BLACKFIN) || defined(CONFIG_NDS32) || \
		defined(CONFIG_SPARC)
	timer_init,		/* initialize timer */
#endif
#ifdef CONFIG_SYS_ALLOC_DPRAM
#if !defined(CONFIG_CPM2)
	dpram_init,
#endif
#endif
#if defined(CONFIG_BOARD_POSTCLK_INIT)
	board_postclk_init,
#endif
#if defined(CONFIG_SYS_FSL_CLK) || defined(CONFIG_M68K)
	get_clocks,
#endif
	env_init,		/* initialize environment */
#if defined(CONFIG_8xx_CPUCLK_DEFAULT)
	/* get CPU and bus clocks according to the environment variable */
	get_clocks_866,
	/* adjust sdram refresh rate according to the new clock */
	sdram_adjust_866,
	init_timebase,
#endif
	init_baud_rate,		/* initialze baudrate settings */
	serial_init,		/* serial communications setup */
	console_init_f,		/* stage 1 init of console */
#ifdef CONFIG_SANDBOX
	sandbox_early_getopt_check,
#endif
#ifdef CONFIG_OF_CONTROL
	fdtdec_prepare_fdt,
#endif
	display_options,	/* say that we are here */
	display_text_info,	/* show debugging info if required */
#if defined(CONFIG_MPC8260)
	prt_8260_rsr,
	prt_8260_clks,
#endif /* CONFIG_MPC8260 */
#if defined(CONFIG_MPC83xx)
	prt_83xx_rsr,
#endif
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
	checkcpu,
#endif
	print_cpuinfo,		/* display cpu info (and speed) */
#if defined(CONFIG_MPC5xxx)
	prt_mpc5xxx_clks,
#endif /* CONFIG_MPC5xxx */
#if defined(CONFIG_DISPLAY_BOARDINFO)
	show_board_info,
#endif
	INIT_FUNC_WATCHDOG_INIT
#if defined(CONFIG_MISC_INIT_F)
	misc_init_f,
#endif
	INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_HARD_I2C) || defined(CONFIG_SYS_I2C)
	init_func_i2c,
#endif
#if defined(CONFIG_HARD_SPI)
	init_func_spi,
#endif
	announce_dram_init,
	/* TODO: unify all these dram functions? */
#if defined(CONFIG_ARM) || defined(CONFIG_X86) || defined(CONFIG_NDS32) || \
		defined(CONFIG_MICROBLAZE) || defined(CONFIG_AVR32)
	dram_init,		/* configure available RAM banks */
#endif
#if defined(CONFIG_MIPS) || defined(CONFIG_PPC) || defined(CONFIG_M68K)
	init_func_ram,
#endif
#ifdef CONFIG_POST
	post_init_f,
#endif
	INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_SYS_DRAM_TEST)
	testdram,
#endif /* CONFIG_SYS_DRAM_TEST */
	INIT_FUNC_WATCHDOG_RESET

#ifdef CONFIG_POST
	init_post,
#endif
	INIT_FUNC_WATCHDOG_RESET
	/*
	 * Now that we have DRAM mapped and working, we can
	 * relocate the code and continue running from DRAM.
	 *
	 * Reserve memory at end of RAM for (top down in that order):
	 *  - area that won't get touched by U-Boot and Linux (optional)
	 *  - kernel log buffer
	 *  - protected RAM
	 *  - LCD framebuffer
	 *  - monitor code
	 *  - board info struct
	 */
	setup_dest_addr,
#if defined(CONFIG_BLACKFIN)
	/* Blackfin u-boot monitor should be on top of the ram */
	reserve_uboot,
#endif
#if defined(CONFIG_SPARC)
	reserve_prom,
#endif
#if defined(CONFIG_LOGBUFFER) && !defined(CONFIG_ALT_LB_ADDR)
	reserve_logbuffer,
#endif
#ifdef CONFIG_PRAM
	reserve_pram,
#endif
	reserve_round_4k,
#if !(defined(CONFIG_SYS_ICACHE_OFF) && defined(CONFIG_SYS_DCACHE_OFF)) && \
		defined(CONFIG_ARM)
	reserve_mmu,
#endif
#ifdef CONFIG_DM_VIDEO
	reserve_video,
#else
# ifdef CONFIG_LCD
	reserve_lcd,
# endif
	/* TODO: Why the dependency on CONFIG_8xx? */
# if defined(CONFIG_VIDEO) && (!defined(CONFIG_PPC) || defined(CONFIG_8xx)) && \
		!defined(CONFIG_ARM) && !defined(CONFIG_X86) && \
		!defined(CONFIG_BLACKFIN) && !defined(CONFIG_M68K)
	reserve_legacy_video,
# endif
#endif /* CONFIG_DM_VIDEO */
	reserve_trace,
#if !defined(CONFIG_BLACKFIN)
	reserve_uboot,
#endif
#ifndef CONFIG_SPL_BUILD
	reserve_malloc,
	reserve_board,
#endif
	setup_machine,
	reserve_global_data,
	reserve_fdt,
	reserve_arch,
	reserve_stacks,
	setup_dram_config,
	show_dram_config,
#if defined(CONFIG_PPC) || defined(CONFIG_M68K) || defined(CONFIG_MIPS)
	setup_board_part1,
#endif
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
	INIT_FUNC_WATCHDOG_RESET
	setup_board_part2,
#endif
	display_new_sp,
#ifdef CONFIG_SYS_EXTBDINFO
	setup_board_extra,
#endif
	INIT_FUNC_WATCHDOG_RESET
	reloc_fdt,
	setup_reloc,
#if defined(CONFIG_X86) || defined(CONFIG_ARC)
	copy_uboot_to_ram,
	clear_bss,
	do_elf_reloc_fixups,
#endif
#if !defined(CONFIG_ARM) && !defined(CONFIG_SANDBOX)
	jump_to_copy,
#endif
	NULL,
};

之後返回crt0.S，繼續執行後續函式，如board_init_r在common/Board_r.c中定義

7、Board_r.c：board_init_r

檔案位置：common/Board_r.c

void board_init_r(gd_t *new_gd, ulong dest_addr)
{
#ifdef CONFIG_NEEDS_MANUAL_RELOC
	int i;
#endif

#ifdef CONFIG_AVR32
	mmu_init_r(dest_addr);
#endif

#if !defined(CONFIG_X86) && !defined(CONFIG_ARM) && !defined(CONFIG_ARM64)
	gd = new_gd;
#endif

#ifdef CONFIG_NEEDS_MANUAL_RELOC
	for (i = 0; i < ARRAY_SIZE(init_sequence_r); i++)
		init_sequence_r[i] += gd->reloc_off;
#endif

	if (initcall_run_list(init_sequence_r))
		hang();

	/* NOTREACHED - run_main_loop() does not return */
	hang();
}
	之後呼叫了init_sequence_r中函式在common/Board_r.c同文件中定義。
init_fnc_t init_sequence_r[] = {
	initr_trace,
	initr_reloc,
	/* TODO: could x86/PPC have this also perhaps? */
#ifdef CONFIG_ARM
	initr_caches,
	/* Note: For Freescale LS2 SoCs, new MMU table is created in DDR.
	 *	 A temporary mapping of IFC high region is since removed,
	 *	 so environmental variables in NOR flash is not availble
	 *	 until board_init() is called below to remap IFC to high
	 *	 region.
	 */
#endif
	initr_reloc_global_data,
#if defined(CONFIG_SYS_INIT_RAM_LOCK) && defined(CONFIG_E500)
	initr_unlock_ram_in_cache,
#endif
	initr_barrier,
	initr_malloc,
	initr_console_record,
#ifdef CONFIG_SYS_NONCACHED_MEMORY
	initr_noncached,
#endif
	bootstage_relocate,
#ifdef CONFIG_DM
	initr_dm,
#endif
	initr_bootstage,
#if defined(CONFIG_ARM) || defined(CONFIG_NDS32)
	board_init,	/* Setup chipselects */
#endif
	/*
	 * TODO: printing of the clock inforamtion of the board is now
	 * implemented as part of bdinfo command. Currently only support for
	 * davinci SOC's is added. Remove this check once all the board
	 * implement this.
	 */
#ifdef CONFIG_CLOCKS
	set_cpu_clk_info, /* Setup clock information */
#endif
	stdio_init_tables,
	initr_serial,
	initr_announce,
	INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_NEEDS_MANUAL_RELOC
	initr_manual_reloc_cmdtable,
#endif
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
	initr_trap,
#endif
#ifdef CONFIG_ADDR_MAP
	initr_addr_map,
#endif
#if defined(CONFIG_BOARD_EARLY_INIT_R)
	board_early_init_r,
#endif
	INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_LOGBUFFER
	initr_logbuffer,
#endif
#ifdef CONFIG_POST
	initr_post_backlog,
#endif
	INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_SYS_DELAYED_ICACHE
	initr_icache_enable,
#endif
#if defined(CONFIG_PCI) && defined(CONFIG_SYS_EARLY_PCI_INIT)
	/*
	 * Do early PCI configuration _before_ the flash gets initialised,
	 * because PCU ressources are crucial for flash access on some boards.
	 */
	initr_pci,
#endif
#ifdef CONFIG_WINBOND_83C553
	initr_w83c553f,
#endif
#ifdef CONFIG_ARCH_EARLY_INIT_R
	arch_early_init_r,
#endif
	power_init_board,
#ifndef CONFIG_SYS_NO_FLASH
	initr_flash,
#endif
	INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_PPC) || defined(CONFIG_M68K) || defined(CONFIG_X86) || \
	defined(CONFIG_SPARC)
	/* initialize higher level parts of CPU like time base and timers */
	cpu_init_r,
#endif
#ifdef CONFIG_PPC
	initr_spi,
#endif
#ifdef CONFIG_CMD_NAND
	initr_nand,
#endif
#ifdef CONFIG_CMD_ONENAND
	initr_onenand,
#endif
#ifdef CONFIG_GENERIC_MMC
	initr_mmc,
#endif
#ifdef CONFIG_HAS_DATAFLASH
	initr_dataflash,
#endif
	initr_env,
#ifdef CONFIG_SYS_BOOTPARAMS_LEN
	initr_malloc_bootparams,
#endif
	INIT_FUNC_WATCHDOG_RESET
	initr_secondary_cpu,
#if defined(CONFIG_ID_EEPROM) || defined(CONFIG_SYS_I2C_MAC_OFFSET)
	mac_read_from_eeprom,
#endif
	INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_PCI) && !defined(CONFIG_SYS_EARLY_PCI_INIT)
	/*
	 * Do pci configuration
	 */
	initr_pci,
#endif
	stdio_add_devices,
	initr_jumptable,
#ifdef CONFIG_API
	initr_api,
#endif
	console_init_r,		/* fully init console as a device */
#ifdef CONFIG_DISPLAY_BOARDINFO_LATE
	show_board_info,
#endif
#ifdef CONFIG_ARCH_MISC_INIT
	arch_misc_init,		/* miscellaneous arch-dependent init */
#endif
#ifdef CONFIG_MISC_INIT_R
	misc_init_r,		/* miscellaneous platform-dependent init */
#endif
	INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_CMD_KGDB
	initr_kgdb,
#endif
	interrupt_init,
#if defined(CONFIG_ARM) || defined(CONFIG_AVR32)
	initr_enable_interrupts,
#endif
#if defined(CONFIG_MICROBLAZE) || defined(CONFIG_AVR32) || defined(CONFIG_M68K)
	timer_init,		/* initialize timer */
#endif
#if defined(CONFIG_STATUS_LED)
	initr_status_led,
#endif
	/* PPC has a udelay(20) here dating from 2002. Why? */
#ifdef CONFIG_CMD_NET
	initr_ethaddr,
#endif
#ifdef CONFIG_BOARD_LATE_INIT
	board_late_init,
#endif
#ifdef CONFIG_FSL_FASTBOOT
	initr_fastboot_setup,
#endif
#if defined(CONFIG_CMD_AMBAPP)
	ambapp_init_reloc,
#if defined(CONFIG_SYS_AMBAPP_PRINT_ON_STARTUP)
	initr_ambapp_print,
#endif
#endif
#ifdef CONFIG_CMD_SCSI
	INIT_FUNC_WATCHDOG_RESET
	initr_scsi,
#endif
#ifdef CONFIG_CMD_DOC
	INIT_FUNC_WATCHDOG_RESET
	initr_doc,
#endif
#ifdef CONFIG_BITBANGMII
	initr_bbmii,
#endif
#ifdef CONFIG_CMD_NET
	INIT_FUNC_WATCHDOG_RESET
	initr_net,
#endif
#ifdef CONFIG_POST
	initr_post,
#endif
#if defined(CONFIG_CMD_PCMCIA) && !defined(CONFIG_CMD_IDE)
	initr_pcmcia,
#endif
#if defined(CONFIG_CMD_IDE)
	initr_ide,
#endif
#ifdef CONFIG_LAST_STAGE_INIT
	INIT_FUNC_WATCHDOG_RESET
	/*
	 * Some parts can be only initialized if all others (like
	 * Interrupts) are up and running (i.e. the PC-style ISA
	 * keyboard).
	 */
	last_stage_init,
#endif
#ifdef CONFIG_CMD_BEDBUG
	INIT_FUNC_WATCHDOG_RESET
	initr_bedbug,
#endif
#if defined(CONFIG_PRAM) || defined(CONFIG_LOGBUFFER)
	initr_mem,
#endif
#ifdef CONFIG_PS2KBD
	initr_kbd,
#endif
#if defined(CONFIG_SPARC)
	prom_init,
#endif
#ifdef CONFIG_FSL_FASTBOOT
	initr_check_fastboot,
#endif
	run_main_loop,
};

static int run_main_loop(void)
{
#ifdef CONFIG_SANDBOX
	sandbox_main_loop_init();
#endif
	/* main_loop() can return to retry autoboot, if so just run it again */
	for (;;)
		main_loop();
	return 0;
}

main_loop在common/ main.c中定義

8、main.c： main_loop

common/ main.c

void main_loop(void)
{
	const char *s;

	bootstage_mark_name(BOOTSTAGE_ID_MAIN_LOOP, "main_loop");

#ifndef CONFIG_SYS_GENERIC_BOARD
	puts("Warning: Your board does not use generic board. Please read\n");
	puts("doc/README.generic-board and take action. Boards not\n");
	puts("upgraded by the late 2014 may break or be removed.\n");
#endif

#ifdef CONFIG_VERSION_VARIABLE
	setenv("ver", version_string);  /* set version variable */
#endif /* CONFIG_VERSION_VARIABLE */

	cli_init();

	run_preboot_environment_command();

#if defined(CONFIG_UPDATE_TFTP)
	update_tftp(0UL, NULL, NULL);
#endif /* CONFIG_UPDATE_TFTP */

	s = bootdelay_process();
	if (cli_process_fdt(&s))
		cli_secure_boot_cmd(s);

	autoboot_command(s);

	cli_loop();
}

main_loop中執行延時函式，並決定是否啟動核心，或者解析命令。歡迎繼續閱讀：

點選：

小結：

    uboot的執行流程：

（1）_start（arch/arm/lib/vector.S）
b	reset
（2）reset（arch/arm/cpu/armv7/start.S）
		bl	cpu_init_cp15
		bl	cpu_init_crit
		bl	_main
（3）_main（arch\arm\lib\crt0.S）
		board_init_f (common\Board_f.c）
		b	relocate_code
		ldr	lr, =board_init_r（common/Board_r.c）
			run_main_loop（）
				main_loop()
（4）main_loop（）

例如，uboot中LCD的初始化流程如下所示：

LCD的初始化流程：ENTRY(_main)-》board_init_f-》init_sequence_f-》board_init_r-》init_sequence_r-》stdio_add_devices-》

board_video_skip-》drv_video_init-》